This invention relates generally to radiators of electromagnetic energy and more particularly to loop antennas which utilize a portion of the support structure for a return path for RF current.
Wire antennas for use on military aircraft for air-to-ground communications in the 2 to 30 MHz band is known. Because of limited space the electric length of the conductor of such antennas is only a small fraction of a wavelength. In order to transform the high input impedance of its relatively short length wire to a nominal value of, for example 50 ohms a tuning unit consisting of one or more capacitors is required to be connected between the wire and the radio apparatus. The zig-zag wire antenna currently in use on helicopters is representative of this type of antenna system. The zig-zag configuration is intended to lower the impedance at the input end of the wire in order to reduce the losses in the tuning unit. The electrical performance of this configuration, however, is compromised by the added shunt capacitance between the wire and the aircraft body.
One known attempt to overcome this problem has been disclosed by Cincinnati Electronics in its Model AA-20HF train line antenna which comprises a radiator directly connected at its far end to the aircraft body.